1. Field of the Invention
This invention relates to a communication device and a communication method, and more particularly to a communication device that receives and transmits frames asynchronously via a transmission line, and a communication method executed by the communication device.
2. Description of the Related Art
In a case where data is transmitted and received between two terminal units via a network, data transmitted by one terminal unit is split into frames in a data link layer, and the frames are relayed to the other terminal unit by one or more other communication devices. The communication devices for relaying the frames are implemented e.g. by layer 2 switches.
In general, frame transmission and reception are performed asynchronously between the communication devices. More specifically, a communication device at a transmitting end and a communication device at a receiving end operate in accordance with respective independent clock signals, and hence the transmission of a frame and the reception of the same do not coincide in timing with each other. For this reason, each communication device is provided with a buffer memory for temporarily storing frames so as to accommodate the difference in timing between the transmission and reception of each frame (see e.g. Japanese Unexamined Patent Publication No. 2000-200176). As the standard for an asynchronous network, there may be mentioned Ethernet®, for example.
However, in a case where the operating speed of a receiving-end communication device is lower than that of a transmitting-end communication device, if the utilization factor of a transmission line is high, there is a fear that an overflow of the buffer memory of the receiving-end communication device may be caused by queueing delay or stagnation of frames. To eliminate this fear, there has been proposed a method of controlling intervals at which frames are sent, by the transmitting-end communication device so as to prevent queueing delay or stagnation of frames at the receiving-end communication device (see e.g. Japanese Unexamined Patent Publication No. 2002-271424). More specifically, the transmitting-end communication device transmits frames at proper intervals set in view of the operating speed of the receiving-end communication device. By thus taking into consideration the operating speed of each communication device and the bandwidth of each transmission line at the time of designing a network, it is possible to prevent queueing delay or stagnation of frames at a specific communication device.
By the way, an actual clock frequency of a communication device is not strictly identical to a theoretical clock frequency, because it is physically difficult to generate a clock signal at a frequency strictly identical to a theoretical value thereof. For example, according to the Ethernet standard, the clock has a tolerance of less than 100 ppm (Parts Per Million), i.e. less than 0.01%. Therefore, even if the operating speed of a transmitting-end communication device and that of a receiving-end communication device are theoretically identical to each other, actually, they are not strictly the same.
Conventional communication devices are not provided with any special measures for coping with clock error. Since the clock error is so minute (one clock per 10,000 clocks at the maximum), insofar as the utilization factor of a transmission line leaves room for the full utilization thereof, the queueing delay or stagnation of frames cannot be caused by clock error. However, in a network, such as a trunk network, in which the utilization factor of the transmission line is high, clock error presents a large problem. That is, if the operating speed of a receiving-end communication device becomes slower than that of a transmitting-end communication device due to clock error, frames in transmission queue at the receiving-end communication device progressively increase until an overflow of the memory buffer of the receiving-end communication device is caused.
The overflow of a memory buffer due to clock error cannot be prevented by the above-described technique disclosed in Japanese Unexamined Patent Publication No. 2002-271424. This is because it is difficult to estimate clock error of each communication device when designing a network. Further, whenever there is a change in connecting relations between communication devices, there can be a change in communication devices suffering from queueing delay of frames or in the degree of queueing delay of frames. Furthermore, if the operating speed of a specific communication device is largely changed so as to eliminate queueing delay of frames, this increases frames in transmission queue at other communication devices.